As known, the bottom bracket assembly of a bicycle comprises a shaft and two crank arms associated with the opposite ends of the shaft. The shaft can be made in a separate piece from the crank arms or it can be made in a single piece with one of the two crank arms. The shaft can also consist of two separate pieces (known as half-shafts) that can be coupled together, each of the two half-shafts being made in a single piece with a respective crank arm.
The component of the bicycle bottom bracket assembly consisting of a crank arm and a shaft (or a half-shaft) coupled together or made in a single piece is identified with the expression: crank assembly.
The bottom bracket assembly is mounted on the bicycle housing the shaft in a housing box suitably provided in the bicycle frame. The rotation of the bottom bracket assembly with respect to the bicycle frame is achieved by fitting a pair of rolling bearings onto the shaft. Each bearing is positioned on the shaft at a respective shaft body portion adjacent to the crank arm and, when the bottom bracket assembly is mounted on the bicycle frame, is operatively arranged between the shaft and the housing box provided in the frame.
Due to the load applied by the cyclist on the pedals during pedaling, both the shaft and the crank arms of the bicycle bottom bracket assembly are subjected to high bending stresses. Such stresses, if not suitably counteracted, cause the shaft and/or the crank arms to bend. Such a deformation means a reduction in the efficiency of pedaling and can lead to the shaft and/or the crank arms breaking.
An ever-present demand of bicycle component manufacturers, above all for racing bicycles, is therefore to minimize the bending deformations of the shafts and of the crank arms of the bottom bracket assemblies of bicycles, so as not to alter the efficiency of pedaling and to avoid the risk of components breaking.
Conventionally, the breaking of the shaft and, above all, of the crank arms takes place precisely at the shaft-crank arm coupling zone. Such a zone is therefore a critical zone.